Micro-electro-mechanical system (MEMS) based sensors such as microphones gather information from the environment through measuring physical phenomena. The electronics then process the signal information derived from the sensors despite the presence of noise and parasitic effects. Advantageously, MEMS devices may be manufactured using batch fabrication techniques similar to those used for integrated circuits. Therefore, functionality, reliability, and sophistication may be integrated onto a small silicon chip at a relatively low cost.
MEMS devices may be formed as oscillators, resonators, accelerometers, gyroscopes, pressure sensors, microphone, micro-mirrors, and others. MEMS devices typically use capacitive sensing techniques for measuring the physical phenomenon being measured. In all these applications, the capacitance change of the capacitive sensor is converted into a usable voltage using interface circuits. However, interface circuits may become challenging due to the miniaturization of sensors in the presence of parasitic effects and reduced sense capacitance.
Some of the key characteristics of a MEMS device include sensitivity, bandwidth, linearity, dynamic range, minimum detectable signal, stability, size, and cost. Sensitivity of a MEMS device is the change in the output voltage for an input physical phenomenon (e.g., time varying pressure) derived change in capacitance at the capacitive sensor. Bandwidth is the range of frequencies over which the sensor can be used.
However, another important metric of a capacitive microphone is linearity. The linearity of the sensor is a measure of how close the output versus input calibration curve approximates a straight line at a given frequency. The slope between the input pressure and output voltage provides the sensitivity of the transducer at that frequency. At high input amplitudes the output of the transducer deviates from an ideal straight line. The lower and higher ends of the linear range are determined by both the sensor interface circuit and the sensor. The lower end is limited by system noise such as thermal noise, 1/f noise, and mechanical noise. The higher end of the linear range is determined by structural non-linearities such as spring stiffening or by circuit non-linearities such as clipping.
The dynamic range of a capacitive transducer is defined as the ratio of the maximum and minimum input signal of the linear range. The deviation of the output from the ideal linear curve causes distortion in the microphone output. When the system is excited at a single frequency, distortion may be computed as the minimum input amplitude that causes the output to deviate from linearity by a fixed percentage.
Therefore, one of the challenges relates to the production of MEMS devices and circuits with better functionality and reliability without increasing costs.